Complex organic molecules from extraterrestrial source are expected to have contributed to the Early Earth chemistry. Methylamine (CH3NH2) has already been observed in the interstellar medium (ISM) and is generally related to the formation of glycine, although the latter has not been identified in the ISM yet. In this work, a chemical model for CH3NH2 was investigated, comprising twenty-eight reactions and including reactions involving NH3 and HOOC, aiming to understand the main routes for formation and decomposition of methylamine and also to infer about the chemical behavior of glycine in the ISM. Calculations were performed at the CCSD(T)/aug-cc-pVTZ//M06-2X/aug-cc-pVTZ level and rate coefficients were calculated adopting the Canonical Variational Transition State Theory (CVTST), in the temperature range from 100 to 4000 K, including tunneling effects. Starting from HCN, the preferred pathway for methylamine formation is through consecutive hydrogenation steps, forming CH2N, CH2NH and CH2NH2 intermediates. Considering the decomposition, dissociation into CH3 and NH2 is the most favorable step. NH3 and HCN are common compounds in interstellar ice analogues and react producing NH2 and CH2N through NH2NCH2 and H2NCH2N intermediates. The latter is proposed here and spectroscopic data for any future experimental investigation are given. Finally, an extension to the ISM glycine chemistry is explored and routes to its formation, from the simplest compounds found in interstellar ices, are proposed.